Commitments And Contingent Liabilities (Guaranteed Obligations) (Detail) - Power [Member] - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Loss Contingencies [Line Items]
Face Value of Outstanding Guarantees	$ 1,846	$ 1,806
Exposure under Current Guarantees	108	139
Letters of Credit Margin Posted	134	157
Letters of Credit Margin Received	59	99
Counterparty Cash Margin Deposited	0	0
Counterparty Cash Margin Received	(2)	(1)
Net Broker Balance Deposited (Received)	(6)	57
Other Letters of Credit	61	$ 51
PennEast Natural Gas Pipeline [Member]
Loss Contingencies [Line Items]
Terminated Guarantee obligation following PennEast sale	106
755 MW Gas-Fired Combined Cycle Generating Station [Member]
Loss Contingencies [Line Items]
Face Value of Outstanding Guarantees	$ 21